<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style>
			#outter {
				position: relative;
				/* opacity: 0; */
				width: 500px;
				height: 500px;
				background-color: #ffff00;
			}
			
			.div1,.div2,.div3,.div4 {
				position: absolute;
				border: 1px solid #000000;
				width: 100px;
				height: 100px;
				box-sizing: border-box;
			}

			.div1 {
				left: 10px;
				top: 10px;
				background-color: #ff0000;
			}

			.div2 {
				left: 30px;
				top: 30px;
				background-color: #00ff00;
				z-index: -1;
			}

			.div3 {
				left: 50px;
				top: 50px;
				background-color: #0000ff;
			}
			
			.div4 {
				left: 70px;
				top: 70px;
				background-color: #00ffff;
			}
			
		</style>
	</head>
	<body>
		
		<div id="outter">
			<div class="div1">1</div>
			<div class="div2">2</div>
			<div class="div3">3</div>
			<div class="div4">4</div>
		</div>
	</body>
</html>
